Evidence for muon neutrino oscillation in an accelerator-based experiment

We present results for $\nu_\mu$ oscillation in the KEK to Kamioka (K2K) long-baseline neutrino oscillation experiment. K2K uses an accelerator-produced $\nu_\mu$ beam with a mean energy of 1.3 GeV directed at the Super-Kamiokande detector 250 km away. The data sample is 8.9 $\times 10^{19}$ protons on target. In total, 107 events are observed in Super-Kamiokande; we expect $151^{+12}_{-10}$ if $\nu_\mu$ does not oscillate. The neutrino energy spectrum distortion caused by $\nu_\mu$ oscillation is also seen. The probability that we would observe these results if there is no neutrino oscillation is 0.0050% (4.0$\sigma$).